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Voters across the nation will cast their votes for federal, state and local offices on Nov. 2. But in Iowa, they began voting two weeks ago and can take advantage of early participation through Oct. 30.

In Ohio, voters started casting ballots last Tuesday, and early voting runs until Nov. 1.

Californians can vote by mail and counties begin mailing ballots out Monday.

In West Virginia, where a suddenly competitive U.S. Senate race has emerged, the conservative group American Crossroads announced last week it's working to deliver absentee ballot mailers and make early voting notifications in that state. Early voting begins there on Oct. 13.

By the end of this week, voters in ten states will have started casting ballots. In all, 36 states and the District of Columbia have adopted some form of pre-Election Day voting.

And the trend is growing in terms of numbers and impact. According to the Early Voting Information Center at Reed College in Portland, Ore., about one-fifth of the electorate voted early in 2004, one-quarter did in 2006, and one-third in 2008.

In the 2008 election, 42 percent of California ballots were cast by mail, but that share jumped to 58 percent in the state’s June primary this year.

Early voting and vote by mail are a convenience for voters, but they’re also changing the way campaigns are run.

What early voting meant for Obama in Florida“Campaigns become more expensive, they become more front-loaded,” said Steve Schale, a Democratic strategist in Florida, where early voting begins Oct. 18, but where some counties have already mailed out ballots.

The state director for Barack Obama’s 2008 Florida campaign, Schale was able to assure Obama when he visited the state on the day before Election Day that, due to early voting efforts, the Democrats had already amassed a 358,000 vote lead. (Obama ended up winning Florida by 236,000.)

Election officials report on which people have voted early or have already mailed in their absentee ballots.

In states where people register by party, the campaigns know that, let’s say, 15 percent of registered Democrats in a particular county have voted by Oct. 15. They don't know how they voted, since the votes aren’t counted until after the polls close on Election Day, only that they did vote.

Even in states that don't have party registration, each campaign will have its list of target voters: people identified through phone banking or door-to-door contact, as strong supporters, leaners, lukewarm, etc. As people vote, the campaign checks names off its target list.

“On Election Day campaigns will know exactly who is left out there,” said Schale. “You can narrow your list.”

Early voting helps explain “why you see campaigns going negative earlier because you never know when that voter is making his final decision,” Schale said.

All three Florida Senate candidates, Republican Marco Rubio, Democrat Kendrick Meek and independent (and ex-Republican) Charlie Crist are on Florida television with advertising. According to the Nielsen Company, Crist led his two rivals during the week of Sept. 20 with 879 TV ads in the state’s five major markets, compared to 527 for Rubio and 439 for Meek.

“Charlie Crist has already begun having to go negative on both Meek and Rubio. I’m fairly certain he would have liked to have spent more time talking about his positive independent streak,” Schale said.

House control at stake Florida is also crucial to control of the House of Representatives. The state has six competitive House races, including three Democratic-held seats that are pure tossups, according to the Cook Political Report.

The early vote affects where the national party committees decide to spend money, Schale said. “In Florida, because of the expedited process of voting, all those decisions have to be made earlier.”

The early voting effect is reflected in urgent appeals from candidates and party leaders.

Pleading for donations to what he called his “Early Voting Action Fund,” Democratic Congressional Campaign Committee chairman Rep. Chris Van Hollen told donors in a Sept. 23 e-mail, “With early voting starting this week in battleground states, we can't sit back and let their (the Republicans’) lies take hold.”

In Washington, every county except one uses vote by mail as the sole method of voting. The ballots will be mailed out no later than Oct. 15.

In Washington’s Third Congressional District, rated a tossup race by analysts, Republican candidate Jaime Herrera told supporters in a Sept. 22 e-mail, “In just 3 weeks, ballots will be in mailboxes — and the entire country will be watching to see what happens here in our own backyard!”

Appealing for money, she said, “Every dollar makes a difference in these final days.”

Did the loyalists turn out to vote?It used to be that the “final days” were the ones in the week leading up to Election Day itself. And Election Day was one day in early November, not a four-or five-week time frame.

Traditionally on Election Day, campaigns stationed workers at precincts where they checked voter rolls to see which of their loyal supporters and "leaners" had voted. If some failed to show up within a few hours of poll closing time, workers alerted campaign headquarters and phone calls went out to spur the laggards to hustle to their precincts and vote.

That kind of voter tracking is still vital, but early voting allows campaigns to target their appeals — which doors volunteers knock on and which households mail is sent to — more efficiently.

“We’re using it as a tool to increase turnout among unlikely voters,” said Iowa Democratic Party spokesman Sam Roecker. “We’re reaching out to voters who otherwise wouldn’t vote, expanding the electorate, rather than switching voters from one of method of voting to the other.”

Iowa Republican Party executive director Jim Anderson said the GOP is mounting “the largest and most aggressive early vote program the party has ever put together.”

Maximize turnout through satellite locations Each party in Iowa can petition county auditors in each county to open satellite voting locations. Each side does that in a strategic way to maximize turnout in places where they’re strong.

“We looked at the past couple of election cycles where turnout was higher for Republicans,” Anderson said. “We overlaid our target maps for the (state) House districts and the Senate districts where we think we’re going to need a couple of extra votes ... We’d like a satellite location in a heavily populated Republican precinct.”

One risk of an early election is that a candidate who is behind may be tagged by the last week of September as a likely loser and be unable to catch up.

The Nielsen Company reported that “the Portman campaign dominated Fisher in paid advertising presence" the week of Sept. 20, running over five times as many ads in Ohio’s three major markets.” Fisher ran no ads at all in the Cincinnati market.

Can candidates catch up? Fisher “may never receive enough funds to counter Portman's spending, but if he does eventually increase his resources, there may be relatively few voters who haven't cast a ballot yet,” said John Green, director of the Bliss Institute of Applied Politics at the University of Akron. But Green also said, “Early voters tend to be the strongest partisans, so Fisher will get his share of the early votes among Democrats.”

An early ad blitz has some liabilities.

If half of the voters wait until Nov. 2 to vote, then those people might have forgotten the ads or campaign mail pieces they saw in the second week of October.

A campaign might run the risk of focusing too much of its ad barrage on the early deciders.

“The earlier ads will be less fresh or ‘accessible’ than the later ones” in the minds of those who don’t vote until Nov. 2, said political scientist Paul Gronke, director of the Early Voting Information Center.

“The smart campaign will not launch all of its artillery ‘early,’ and will focus early ads on the demographic groups (older, wealthier, more partisan, more ideological people) that tend to vote early,” Gronke said. “Later ads will remind voters of the salient points in the earlier ads.”

He added, “It's as if the whole advertising timeline is stretched out from what was a seven-day ad blitz to a 70-day ad blitz.”

And if you voted early, you can ignore those last-minute ads and just wait for the votes to be counted.